1. Field of the Invention
The present invention relates to a linear actuator, and production method and inspection method of a linear actuator.
2. Description of the Related Art
A liner actuator, which has its output shaft reciprocating in an axial direction, is recently heavily used in various equipments as a means for generating a linear movement working directly on an object. A linear actuator, in which the travel distance of an output shaft can be precisely controlled, is disclosed in, for example, Japanese Patent Application KOKAI Publication No. 2002-122203 (refer to Paragraphs [0015] to [0021], and FIG. 1 therein).
FIG. 4 is a schematic cross sectional view of a conventional linear actuator. The linear actuator uses a stepping motor which generally comprises a stator assembly 10 and a rotor assembly 20.
The stator assembly 10 includes two stator units, one of which comprises a pair of stator yokes 13a, 13b disposed so as to oppose each other and to sandwich a bobbin 12 having a winding 11 provided therearound, and the other of which comprises a pair of stator yokes 16a, 16b disposed so as to oppose each other and to sandwich a bobbin 15 having a winding 14 provided therearound. The stator yokes 13a, 13b each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. In the same way, the stator yokes 16a, 16b each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing each other. The windings 11, 14 are responsible for exciting respective pole teeth of the two stator units. The two stator units structured as described above are coaxially stacked on each other, thus forming the stator assembly 10. The stator yokes 13a, 13b, and 16a, 16b, and the bobbin 12, 15, which are constituent members of the stator assembly 10, are fixed together by means of resin injected. Thus, the stator assembly 10 is shaped substantially hollow-cylindrical and looks like a doughnut, in which the arrays of the pole teeth of the stator yokes 13a, 13b, and 16a, 16b define the inner circumference of the stator assembly 10.
The rotor assembly 20 is housed in the hollow of the cylindrically shaped stator assembly 10. The rotor assembly 20 is composed of a rotor magnet 21 having a plurality of magnetic poles, a resin segment 22 and a female screw 23, and is shaped substantially hollow-cylindrical. The rotor magnet 21 is shaped in a ring, and constitutes the outer circumference of the rotor assembly 20 so as to oppose the pole teeth defining the inner circumference of the stator assembly 10. The resin segment 22 is shaped in a tube and fixedly disposed inside the ring-shaped rotor magnet 21, and the female screw 23 is fixedly disposed inside the tube-shaped resin segment 22.
The stator assembly 10 has a rear end cap 30 formed at its rear end face so as to cover the hollow of the stator assembly 10. The rear end cap 30 is formed of resin which is injected simultaneously at the process of resin injection-molding performed to fix together the constituent members of the stator assembly 10. The rear end cap 30 has a circular cavity 31 on an inner side thereof facing the rotor assembly 20, and a rear ball bearing 32 is fitted into a circular recess which is formed on the inner side of the rear end cap 30 so as to be concentric with the cavity 31. The rear ball bearing 32 rotatably supports the rear end of the rotor assembly 20.
The rearward portion (toward a rear end 40b) of an output shaft 40 is movably inserted through the hollow-cylindrical rotor assembly 20, and is threaded thereby forming a male screw 41, which engages threadedly with the female screw 23 disposed inside the rotor assembly 20.
The stator assembly 20 has a front end cap 50 attached to its front end face. The front end cap 50 has a center hole 51, and a groove 54 formed on its inner circumference so as to extend along the longitudinal direction of the output shaft 40. The frontward portion (toward a front end 40a) of the output shaft 40 goes movably through the center hole 51 so as to protrude from the front end cap 50. The front end cap 50 further has a circular recess 52 formed on its inner side so as to be concentric with the center hole 51, and a front ball bearing 53 is fitted into the recess 52. The ball bearing 53 rotatably supports the front end of the rotor assembly 20.
The output shaft 40 has a stopper pin 42 formed at an area of its frontward portion within the extent of the groove 54 of the front end cap 50. The stopper pin 42 prohibits or restricts rotation of the output shaft 40, whereby the output shaft 40 is put into a linear motion in an axial direction when the rotor assembly 20 rotates.
In the linear actuator described above with reference to FIG. 4, when current is caused to flow in the windings 11, 14, the pole teeth of the stator assembly 10 are excited thereby causing the rotor assembly 20 having the rotor magnet 21 to rotate with respect to the stator assembly. The rotary motion of the rotor assembly 20 is converted into a linear motion of the output shaft 40, for example, in a frontward direction by way of the female screw 23 and the male screw 41 threadedly engaging with each other, and when the rotational direction of the rotor assembly 20 is reversed, the output shaft 40 is linearly moved in a reverse direction, that is, in a rearward direction.
In the linear actuator shown in FIG. 4, while the rear end cap 30 can be precisely aligned coaxially with the stator assembly 10 thanks to the rear end cap 30 being formed by the aforementioned simultaneous resin injection-molding process performed to produce the stator assembly 10, the front end cap 50, which is press-fitted into the hollow of the stator assembly 10, is not necessarily aligned coaxially with the stator assembly 10 depending on the dimensional accuracy of relevant components thus failing to ensure an assembly accuracy.
Also, for ensuring the quality of the linear actuator, the rotor assembly 20 is rotated to check noises generated thereby for the purpose of evaluating the rotation characteristic of the rotor assembly 20 with respect to the stator assembly 10. In this connection, the rotor assembly 20 can be duly rotated only after the front end cap 50 with the front ball bearing 53 fixed thereto is attached to the stator assembly 10 where the output shaft 40 is put through the center hole 51 of the front end cap 50 with its rearward portion positioned inside the rotor assembly 20. The rotation characteristic of the rotor assembly 20 with respect to the stator assembly 10 is evaluated by checking noises generated by the rotation, but the noises are drowned out by other noises generated by the female screw 23 and the male screw 41 threadedly engaging with each other thus hindering an accurate evaluation.
Further, since the travel amount of the output shaft 40 allowed for the axial direction is limited, the output shaft 40 cannot be moved continuously in one same direction, which means that the rotor assembly 20 cannot be rotated continuously in one same direction. This makes the evaluation by noises increasingly difficult and troublesome.